This invention generally relates to particle impact collectors, and more specifically, to a coating for a surface used in connection with a particle or aerosol impact collector.
It is often necessary to separate particulates from air or other fluids in which the particulates are entrained. For example, aerosols comprising small droplets of liquid dispersed into air are not easily analyzed unless the aerosol materials are separated from the air to produce a more concentrated sample that can then be analyzed. The aerosols or particulates can be liquids, solids, or semi-solids that are sufficiently small to be dispersed within and carried about in air and may include inorganic or organic chemicals, or living materials, e.g., bacterial cells or spores. Also, solids or semi-solids can be introduced into a liquid that is then dispersed within air as an aerosol mist so that the solids are carried within the liquid droplets comprising the aerosol mist.
Generally, it is difficult to identify materials comprising particulates entrained in a gaseous fluid unless the particulates can be collected by separating them from the air or other gaseous fluid and concentrated in a specimen suitable for analysis. In addition to identifying the type of particulates entrained in a gaseous fluid, it may be important to classify the size of the particulates. For example, when checking stack emissions, it is usually important to determine the materials carried as particulates within the emissions and the size of the particulates to determine whether the emissions conform to pollution control limits.
Particle impact devices are commonly used for collecting particulates from gaseous streams in which they are dispersed. Conventional particle impactor collectors employ circuitous paths with many abrupt changes of direction along the passages through which a particulate laden fluid flows. The particulates, being substantially more massive than the molecules of the fluid in which they are entrained, fail to negotiate the abrupt turns in these passages and are thus separated from the moving fluid stream, collecting on the surfaces that they impact. To function properly, such prior art particle impactors require that the gaseous fluid stream be moved through the impactor at least at some minimum velocity. A fan is typically used to provide the required velocity to the fluid flowing into the particle impact collector. One problem with particle impact collectors of this type is that it is often difficult to separate the particulates collected from the surfaces on which they have impacted. Furthermore, many of the particulates do not collect on the desired collection surfaces and are therefore unavailable for analysis and evaluation.
Another type of prior art particle impact collector includes a rotating arm that is placed within the flow path of a fluid in which particulates are entrained. The particulates impacting the rotating arms are separated from the fluid and subsequently stripped from the surfaces of the arms. However, such collectors do not provide a simple and efficient mechanism to remove the particulates from the rotating arms.
Virtual impactors are another type of prior art device used for separating particulates from a gaseous fluid, again using the differences in mass of the particulates and the fluid molecules to facilitate the separation process. In this type of device, the gaseous fluid is directed along a passage and separated by a divider disposed within the passage into a fast moving major stream and a much slower moving minor stream. The more massive particulates remain in the slower moving minor stream, while the fluid and very small particulates continue through the device in the major stream. However, virtual impactors simply separate the streams, but do not provide a specific collection mechanism for separating the particulates from the fluid in which they are carried.
Although other types of particle impact collectors and virtual impactors are described in the prior art, none of them employ a coating element on the impactor surface to enhance the efficiency with which particulates entrained in a gaseous fluid are collected. Further, none of these prior art devices provide an efficient mechanism for releasing the particulates collected on the impactor surfaces so that the particles may be analyzed. Clearly, it would be desirable to increase the collection efficiency of a particulate impactor, both in regard to the separation of particulates from the gaseous fluid in which they are entrained, and in connection with removal of the particulates from the surfaces of the device on which they have collected.
The present invention relates to a method for separating particulates from a fluid in which the particulates are entrained. The method includes the steps of providing an impact collection surface and applying a coating of a material to the surface. This material increases the retention of the particulates impacting the material coating the impact collection surface when the material is dry, but readily releases the particulates that have been collected thereon when the material is wetted. The fluid in which the particulates are entrained is caused to flow toward the coated impact collection surface, so that the particulates are retained by the material and are thus separated from the fluid.
In one embodiment, the impact collection surface comprises a surface of a tape. The coating is applied to selected portions of the surface of the tape to produce the coated impact collection surface. The method further includes the step of advancing the tape, so that new areas that are coated with the material are impacted by the particulates entrained in the fluid. In another form of the invention, the impact collection surface includes projecting structure that enhances the collection of particulates. For example, the projecting structure may comprise a plurality of ridges or rods that are relatively small and extend outwardly.
In one form of the invention, the material comprising the coating has the property of attracting only particulates of a specific desired type. For example, the desired type of particulates can be biological in nature. In this case, the coating comprises an antibody selected so that only particulates having a corresponding antigen are retained by the coating. It is thus contemplated that the particulates retained by the coating may comprise a disease causing organism.
In another embodiment, the material comprising the coating is characterized by having a first state and a second state. When the material is in its first state, it attracts the particulates, but when it is in the second state, it releases the particulates. The method thus further comprises the step of causing the material to be in the first state to separate the particulates from the fluid. After a defined period of time has elapsed during which the particulates are separated from the fluid and retained by the coating, the method also includes the step of causing the material comprising the coating to change from its first state to its second state. The particulates that have been separated from the fluid and then released from the material when the material changes to its second state and collected. The step of collecting preferably includes the steps of directing a liquid toward the coated impact collection surface, so that the particulates separated from the fluid are carried away from the surface by the liquid, and collecting the liquid carrying the particulates. In one form of the invention, the step of causing the material to change from the first state to the second state comprises the step of wetting the material with the liquid. In this form of the invention, the material preferably comprises TETRAGLYME, and the liquid preferably comprises water. TETRAGLYME is a common chemical name for bis(2-[methoxyethoxy]ethyl) ether tetraethylene glycol dimethyl ether dimethoxy tetraethylene glycol.
Another type of material used for the coating is characterized by having a relatively low coefficient of friction, so that the particulates initially retained thereon by impacting the coating readily detach from the coating.
The method may also include the step of directing a liquid towards the coating to wash away the particulates that have been separated from the fluid. The liquid and the particulates washed away from the coated impact collection surface are then collected. The liquid preferably comprises water.
Another aspect of the present invention is directed to a method for removing pathogens from a fluid. This method includes the step of providing a particle impact collector having a surface coated with a material that is in either a first or a second state. The first state of the material is characterized by its ability to retain the pathogens and thereby separate them from the fluid. The second state of the material is characterized by its ability to readily release the pathogens. The method includes the step of causing the material to be maintained in the first state. The fluid in which the pathogens are entrained is directed toward the surface, so that the pathogens are retained by the material coating the surface while it is in the first state. These pathogens are thereby removed from the fluid to purify it. After an interval of time in the first state, the state of the material coating the surface is changed to the second state. The pathogens retained by the coating are then released, thereby cleansing the surface so that it may be reused to again purify the fluid.
Yet another aspect of the present invention is directed to apparatus for separating particulates from a fluid in which they are entrained. The various embodiments of the apparatus include elements that are generally consistent with the steps of the method discussed above.